garden is the name of a medicine. Also, it is a word used in a derogatory way, and it means that a person is crazy.
Quite popular on social networks like Twitter, gardenal is a term used to play with friends when they do something crazy, or to insult a person when he does or says something absurd. For example: Did you see what she did? She can only be gardenal! / He's talking like that because he forgot to take the gardenal.
This sense of "madness" was attributed to this word because it is the name of a medicine. Despite this, whoever takes or has taken gardenal is not crazy, this interpretation is the result of prejudice.
Gardenal, whose active ingredient is phenobarbital, is a drug of the barbiturate class, a tranquilizer used in the treatment and prevention of seizures in people and animals. Gardenal is not so common these days, but it used to be prescribed by doctors to people with epilepsy. This medicine acts on the central nervous system and can be a treatment for anxiety.
The association between gardenal and madness is made by many people and may originate from the lack of understanding of some symptoms, such as seizures. In the past, people who had seizures were considered crazy, or were treated as if they were possessed by evil spirits.